Along with the development of the economy and the society in China, more and more resources are consumed, thus a huge pressure is put on the environment. To solve that problem, distributed power sources, mainly renewable energy resources, are being developed vigorously in China. As many distributed power sources are grid-connected, the electric grid suffers impacts. Micro-grids can solve the problems related with grid connection of distributed power sources satisfactorily. However, when a micro-grid operates offline, as the system has small capacity and poor harmonic current endurance capacity, when offline micro-grid starts a large-scale power load, the surge current and harmonic current produced may result in abnormal situation in the system. In order to effectively improve the economic efficiency of micro-grid, it is urgent to develop a method for starting a large-scale power load in a low-capacity micro-grid during offline operation of the micro-grid.
Among the methods for starting a large-scale power load, in the voltage soft start method, a voltage soft starter is utilized, but the starting current is high, and the harmonic current is severe; in addition, during the startup process, the output torque of the large-scale power load is small. Utilizing a variable frequency starter can effectively improve the output torque of the power load and greatly reduce the starting current. However, owing to the fact that a variable frequency starter has an internal rectifier circuit, the harmonic current is high when such a variable frequency starter is used. Since a micro-grid has low capacity and low harmonic current endurance capacity, the harmonic current produced when a power load is started with a variable frequency starter may influence the stability of the micro-grid.